The BGS Lexicon of Named Rock Units — Result Details

Faringdon Sand Formation

The formation is now divided from the base into the Faringdon Sponge Gravel Member, Baulking Sand Member and the Fernham Sand Member. The Faringdon Sponge Gravel Member comprises cross-bedded pebbly sands and highly fossiliferous gravels composed predominantly of the remains of calcareous sponges, with brachiopods, echinoderms and bryozoa. Bored mudstone and limestone clasts and phosphatised fossils, derived from the underlying Kimmeridge Clay and Corallian Group strata, occur at the base of the formation and at higher levels within the succession. Two subdivisions,comprising lower "yellow gravel" and upper (iron-oxide stained) "red gravel", have previously been recognized, separated by a non-sequence. According to Poole et al. (1971), cross-bedding diminishes upwards from the "yellow gravel" into the "red gravel", and there is a change in current direction as indicated by apparent dips. The Baulking Member comprises fine- to medium-grained siltstone and sandy mudstone with horizons of fuller's earth. Locally richly glauconitic with some horizons of shelly sand. The lower part of the interval is usually sandier, with horizons of fuller's earth and slumped beds. The upper part of the formation is mostly dark grey, thin current-bedded mudstone, with ironstone locally developed in the lower part of the interval. The mudstone contains white, fine-grained sandy and silty wisps and laminae. The Ferham Sand Member comprises reddish-brown, ferruginous, cross-bedded, shell-fragmental, medium- to coarse-grained, clean sandstone. There are scattered pebbles, bands of conglomeratic grit and minor horizons of mudstone.

Definition of Lower Boundary:

Unconformable, channelled, at the basal contact of the "yellow gravel" with underlying Kimmeridge Clay mudstones or, locally, Corallian Group limestones.

Definition of Upper Boundary:

Not seen. The Fernham Sand Member occurs as hill-cappings in the Baulking area. The top of the formation is locally defined by the depth of the present-day weathered zone.

Thickness:

Variable depending on local superposition of constituent members that have maximum thicknesses of about 50m (Faringdon Sponge Gravel Member), up to 48m (Baulking Sand Member) and at least 24m (Fernham Sand Member).

Geographical Limits:

In the Faringdon, Baulking to Uffington areas, and around Little Coxwell and Fernham, Oxfordshire.

The Fernham Boreholes (Nos 1 to 14; SU29SE 1 to 9 and SU39SW 13 to 17) and Baulking Boreholes (Nos 1 to 11; SU39SW 2 to 4 and 24 to 31) [SU 28 29 to SU 32 90] (see Poole and Kelk, 1971; Poole et al., 1971 for details). These contain together the full succession of the Fernham Sand and Baulking Sand members

Type Area

In the vicinity of Furze Hill [SU 29 92] and Fernham [SU 293 919], Oxfordshire. Where the upper part of the succession is indicated within field brash and exposures.

Partial Type Section

Grid reference [SU 288 943] is for the pit labelled as "Faringdon Pit" by Arkell (1947). The formation is found in old gravel pits between Faringdon and Little Coxwell, Oxfordshire. At the Faringdon pit only the Sponge Gravel Member is seen.

Type Area

In the vicinity of Baulking [SU 31 90], Oxfordshire. Where the upper part of the succession is indicated within field brash and exposures.

Reference(s):

Austen, R A C. 1850. On the age and position of the fossiliferous sand and gravel of Farringdon. Quarterly Journal of the Geological Society of London, Vol.6, 454-478.